The present invention concerns a drive system and a method of driving a vehicle using the system.
To use a conventional clutch mechanism which disconnects the input shaft of the gear box from the combustion engine during gear changing processes in the gear box involves disadvantages. When a stationary vehicle starts, the discs of the clutch mechanism slide against each other, thereby heating the discs. This heating results in increased fuel consumption and wear of the clutch discs. A conventional clutch mechanism is also relatively heavy and expensive. It occupies also a relatively large space in the vehicle. To use a hydraulic moment converter also results in losses.
Hybrid vehicles can be driven by a primary motor which can be a combustion engine and a secondary motor which can be an electric machine. The electric machine is equipped with at least one energy storage for storing electric energy and control equipment for controlling the flow of electric energy between the energy storage and the electric machine. The electric machine can thereby alternately work as a motor or a generator in dependence on the operation state of the vehicle. When the vehicle is braked, the electric machine generates electric energy which is stored in the energy storage. The stored electric energy is used later, for example, for driving the vehicle and operating different auxiliary systems in the vehicle.
Swedish patent application SE 1051384-4, which has not been made public, shows a hybrid drive system with a planetary gear which comprises three components, namely a sun wheel, a planet wheel holder and a ring wheel. One of the three components of the planetary gear is connected to an output shaft of the combustion engine, a second component of the planetary gear is connected to an input shaft to the gear box and a third component of the planetary gear is connected to a rotor of an electric machine. The electric machine is connected to an energy storage such that it alternately can work as a motor or a generator. The rotation speed of electric machines can be controlled in a stepless manner. By controlling the rotation speed of the electric machine, the input shaft to the gear box can be given a desired rotation speed. With a hybrid system according to SE 1051384-4 no clutch mechanism needs to be used in the drive line of the vehicle.
The moment of the combustion engine and the moment of the electric machine provide together the driving moment of the vehicle. The combustion engine is however often used for the operation of different assemblies in the vehicle. There is also an uncertainty concerning the design of components included in the combustion engine, wear aging, etc. The quality of the fuel, the temperature of the fuel and the temperature of the combustion engine are other factors which make it difficult to determine the moment which the combustion engine supplies for the operation and thereby the driving moment of the vehicle. It is however important that the combustion engine provides a demanded moment with a high accuracy at certain operation states, for example, when gears are shifted and when disengaging the planetary gear.